Add a new assets:precompile wrapper that doesn't GC
Add a new assets:precompile wrapper that doesn't GC

@h-lame has a theory that GC might be one of the reasons that the
stack-based parser in SCSS is failing on systems under load.

This adds a new Rake task called `assets:gc_precompile` that turns off
garbage collection and runs the original `assets:precompile` task.

hotvulcan/whitehall,alphagov/whitehall,ggoral/whitehall,ggoral/whitehall,alphagov/whitehall,robinwhittleton/whitehall,ggoral/whitehall,askl56/whitehall,hotvulcan/whitehall,askl56/whitehall,YOTOV-LIMITED/whitehall,robinwhittleton/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,hotvulcan/whitehall,askl56/whitehall